The present invention relates to ground fault detection circuits and more particularly, to testing ground fault detection circuits using a stimulus signal generated using a low voltage DC power supply.
Ground Fault Circuit Interrupt (GFCI) devices detect the presence of ground current faults and grounded neutral faults, and interrupt power in AC power systems if such faults are detected. Accordingly, GFCI devices provide protection from electrocution and are typically used in receptacles in kitchens, bathrooms, and outdoor receptacles where there may be water or moisture that can pose a risk of electrocution. GFCI devices are also used in circuit breakers that protect these same areas of residential buildings. GFCI devices and other devices that detect ground faults and interrupt AC power systems may also be referred to generally as “ground fault detectors”. Ground fault detectors typically have supervisory circuits or test circuits that check the functionality of the ground fault detection circuit.
Ground fault detectors disconnect a circuit when current leakage is detected. Current leakage occurs when current flowing through a line, or “hot” conductor, from a source load is diverted to ground without returning to the source. This leakage may result from an accidental short circuit, such as from a defective load attached to the line. If a person touches the load, the leakage current may pass through the person's body to ground, leading to an electric shock. Consequently, ground fault detectors, or GFCIs, act as safety devices and are designed to detect line-to-ground shorts and disconnect the distribution circuit.
Ground fault detectors also need to act quickly. While a typical circuit breaker interrupts a circuit at 20 amperes, it only takes approximately 100 milliamperes to electrocute a person. Therefore, for added safety, ground fault detectors must be able to detect current flow between a line and ground at current levels as little as 6 milliamperes and trip a breaker at the receptacle or at the breaker panel to remove the shock hazard. Ground fault detectors are typically required for receptacles in bathrooms and other areas exposed to water in order to prevent deadly ground fault situations from occurring.
In two-line systems, GFCIs typically detect current leakage by comparing the current flowing in the line and returning in the neutral. A difference in current levels implies that some current has leaked from the circuit and a ground fault exists. GFCIs typically use a differential transformer to detect a difference in the current levels in the line and the neutral. The differential transformer is often a toroidal core that has as its primary windings the line and neutral conductors of the distribution circuit being protected, which are encircled by the core. The secondary windings of the transformer are wrapped around the core. During normal conditions, the current flowing in one direction through the line conductor will return in the opposite direction through the neutral conductor. This balance produces a net current flow of zero through the differential transformer, and the multi-turn winding provides no output. If a fault exists, current leaks from the line conductor to ground and the current flowing back through the line and neutral conductors in the differential transformer will not be equal. This current imbalance will produce uncanceled flux in the differential transformer's core, resulting in an output from the multi-turn secondary winding. Detection circuitry identifies the output from the differential transformer and opens the circuit breaker contacts.